1. Field of the Invention
The present invention relates to resistor elements and more particularly to a method of forming multiple precision resistor elements on integrated circuits by employing a circuit utilizing at least one MOS transistor element.
In recent years, new and improved methods have been sought for forming precision resistors on integrated circuits. Applications in which such precision resistors may be used include filter circuits, precision current mirrors, and analog-to-digital and digital-to-analog converters, to name but a few.
2. Description of the Prior Art
Several techniques are known for forming precision resistors on integrated circuits in the environment of filters. For example, switched capacitor filters, commonly referred to as switched-C filters, are known which take advantage of the resistance characteristics exhibited by on-chip capacitors in order to form high and low pass filters. In switched-C filters, the terminals of a capacitor are alternately switched between corresponding nodes of the circuit. An alternating charge is produced which is proportional to the difference in voltage at the corresponding nodes. The alternating charge produces a current similar to that which would result if a resistor were placed between the corresponding nodes of the circuit. The current developed is proportional to the voltage difference between the nodes, enabling the capacitor to be utilized as a resistor. The resistive elements realized by the switched capacitor resistance forming technique may be utilized in other circuits such as digital-to-analog converters and analog-to-digital converters where a plurality of devices are slaved.
Switched capacitor filters, however, suffer from several disadvantages. First, an independent clock having a very precise frequency must be provided, increasing the cost associated with such devices. Second, the on-chip capacitors and the clock must be chosen such that the resulting filter frequency is less than half of the clock frequency, or the resulting filter will fail to satisfy the Nyquist criteria. Third, the on-chip capacitors which are utilized must be linear. This requires a second layer polysilicon or buried diffusion, or a very large chip area, all of which are seen as undesirable from a design standpoint.
Unswitched capacitor filters which contain on-chip resistors are also known. The on-chip resistors are typically formed as polysilicon or well resistors or by diffusion. Such resistors, however, also suffer from several disadvantages. For example, they are relatively imprecise due to temperature sensitivity, and process tolerances in diffusion. Furthermore, diffused resistors have high voltage sensitivity, and the polysilicon resistors utilize a large die area.
MOS transistors have also been utilized as on-chip resistors. For example, a plurality of on-chip resistive MOS transistors have been formed by connecting their gates to a common voltage or to an external pin, and trimming the external voltage with an external resistive voltage divider. Use of MOS transistors as resistors, however, has also proven to be disadvantageous. First, the effective resistance of MOS transistors is process dependent. As a result, process variations result in inaccurate and variable resistance values. Moreover, when MOS transistor slaving occurs, a very stable power supply voltage must be applied to the gates of the transistors since changes in the common gate voltage may have a very significant effect on the effective ohmic value of the slaved transistors. In addition, MOS parameters which affect the MOS resistance values are significantly temperature dependent, resulting in very limited operational temperature ranges for the resistors produced.